Generally, a semiconductor wafer, such as a silicon substrate, is subjected to various vacuum processes including a film-forming process and an etching process to fabricate a semiconductor integrated circuit. It is very important, from the viewpoint of improving the yield of the product and stabilizing the electrical characteristic the product, particularly for a single-substrate vacuum-processing apparatus that processes a single semiconductor wafer at a time to be capable of forming thin films of the same thickness on successively subjected to the film-forming process one by one, i.e. of maintaining thickness reproducibility.
In processing a semiconductor wafer by such a conventional vacuum-processing apparatus, the semiconductor wafer is mounted, for example, on a susceptor placed in a processing vessel, and heated by a heater to keep the semiconductor wafer at a predetermined temperature. At the same time, a process gas, such as a deposition gas, is supplied at a predetermined gas supply rate into the processing vessel while the processing vessel is being evacuated to maintain a predetermined pressure in the processing vessel. In this state, the film-forming process is continued for a predetermined processing time to process the semiconductor wafer. Process parameters including the processing temperature and the processing pressure in the processing vessel, and the gas supply rates of the process gases are stably maintained by a feedback control operation during the film-processing operation.
Unnecessary substances deposit gradually in unnecessary films on the inner surface of the walls of the processing vessel, the surface of a showerhead through which process gases are supplied, and the surface of the susceptor as the number of processed wafer increases with the continuation of the film-forming process. Consequently, the gradual change of the condition of the processing vessel, such as a reflectivity and a view factor in the processing vessel, is unavoidable. Therefore, although the process parameters are controlled so that the same process conditions, such as the same processing temperature, the same flow rates of the process gases and the same processing pressure, may be maintained, the thickness of the thin film formed on wafers changes (for example, decreases) every time one wafer is processed, so that the thickness reproducibility of the film-forming process deteriorates.
The interior of the processing vessel is cleaned periodically or indeterminately to remove the unnecessary films. Such a mode of change of the thickness of the thin film formed on the wafer varies periodically every time the interior of the processing vessel is cleaned or every time a batch of wafers (for example, twenty-five wafers) is processed. Such a problem may be solved by cleaning the interior of the processing vessel every time one or some wafers are processed. However, excessively frequent cleaning of the interior of the processing vessel decreases the throughput greatly and hence is unpractical.
Enhancement of severity of design rules of the semiconductor integrated circuits and demand for finer lines and thinner films in recent years require the solution of such a problem urgently.